What abstract and/or productive uses do you have, or have had using you linux box?
Run an in-home telephone network linking me to local (in-home) numbers, outside lines, and voip lines. Also, I had incoming numbers in various area codes that when called would perform a task. I used this same concept to stream internet radio through standard PSTN lines and to cell phones at no cost. I used Asterisk PBX, SIPPhone, FWD, and IPKall services.
Utilized the parallel port as a power source and used C++ to code programs that would control homemade robotics. I started with a internet enabled toothbrush, and then used legos and knex motors for more advanced robotics.
Run a web site off my home connection sucking up all my bandwidth. (bad idea)
I am currently:
Running SMB and CCXStream servers to stream media over the network to a modded Xbox running XBMC.
Running nethostfs to stream media over the network to a PSP.
Using proxies, dummy accounts, advanced IM clients, and advanced network mapping tools to bait predators in chatrooms. Do not try this if you don't know what you are doing. If you want to know (and intend to take a serious outlook on it), PM me. It's quite fun baiting those pedophiles.
What do you do that is made easy by using linux?